JP-P2005-522617A (U.S. Pat. No. 7,194,997B2) shows monitoring method for an internal combustion engine in which a current torque is computed based on an actual fuel injection quantity and/or a target fuel injection quantity. When the current torque is larger than a permissible torque, a fuel cut is performed.
In a certain driving condition of the engine, even if fuel injection quantity increases, the actual torque may not increase unless a fuel injection timing is changed. In the above method, since the current torque is computed based on a fuel injection quantity without considering the fuel injection timing, if the fuel injection quantity is increased and actual torque does not increase due to the fuel injection timing, the current torque may become larger than the permissible torque. As the result, it may be determined that the torque is excessively increased and the fuel cut may be performed.
In a direct injection engine, a combustion mode is changed between a homogeneous combustion mode and a stratified combustion mode. Generally, in the homogeneous combustion mode, the fuel injection quantity is determined based on an intake air flow rate which is adjusted by a throttle valve to control the torque. An actual torque is accurately obtained by computing an air-quantity-base actual torque based on an intake air quantity. In the stratified combustion mode, the torque is controlled by adjusting fuel injection quantity in a condition that the throttle valve is fully opened. The actual torque is obtained by computing a fuel-quantity-base actual torque based on the fuel injection quantity.
In the stratified combustion mode, when the fuel injection timing is moved into an intake stroke or its vicinity, the combustion mode is close to the homogeneous combustion even though the combustion mode is the stratified combustion. In such a case, even if the combustion mode is the stratified combustion which is suitable for the fuel-quantity-base actual torque, the computation accuracy of the fuel-quantity-base actual torque is deteriorated so that it cannot be accurately determined whether a torque malfunction exists.